Schluter SET® is the unmodified thin-set mortar that Schluter specifically engineered for use with their membranes. When you're installing DITRA over a concrete slab or bonding tile to a DITRA/DITRA-HEAT membrane, this is what Schluter requires — and using it is what keeps the warranty intact.

Why unmodified matters: The "unmodified" classification means no polymer additives. Polymer-modified mortars can create a slip plane in certain conditions when used between two non-absorptive surfaces (like tile over DITRA membrane) — the polymer never fully cures without absorption. Unmodified SET cures properly every time in that assembly.

Key specs:

• Type: Unmodified thin-set mortar (ANSI A118.1)
• Colors: Grey and White
• Shelf life: 2 years (unopened, not frozen)
• Grout wait: 12 hours at 70°F
• Lifetime warranty when used correctly with Schluter products

When to use SET:

• Bonding DITRA or DITRA-HEAT to concrete
• Bonding DITRA or DITRA-HEAT to gypsum
• Setting tile over any DITRA/DITRA-HEAT membrane
• KERDI installation on most surfaces

When to use ALL-SET instead: If your subfloor is plywood or OSB (wood), use ALL-SET (modified) — wood needs the polymer for adequate bond strength.

Pro Tip: Mix SET to a slightly more fluid consistency when bonding the membrane to the substrate (more water) and a stiffer consistency when setting tile on top (less water). The Schluter installation guide lists specific water ratios — it's worth following them precisely the first time.

Don't substitute a generic unmodified thin-set if you want to maintain the Schluter warranty. The warranty specifically requires Schluter SET, ALL-SET, FAST-SET, or Schluter-listed equivalents.